Ubers is more fun, because you basically get to use whatever the hell you want (besides your 4th-grade Mind Reader + Sheer Cold Articuno). Gimmicky teams are quite fun, especially since the 800-1200 tier range is pretty noobish for Ubers (as opposed to the skill level for the average OU player) and is nice to go up against with trick teams.

UU feels like OU but with a different cast. People use only a few main pokemon again, and it doesn't have much of a creative atmosphere.